1. Field of the Invention
The present invention relates to a backlight, and more particularly, to apparatus and method for driving a backlight. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing malfunction of a shutdown circuit.
2. Discussion of the Related Art
Generally, a liquid crystal display (referred to as “LCD”) has the characteristics of light weight, thin profile, low power consumption. Because of these characteristics, the LCD has been used for many applications. For example, the LCD is used in televisions, computer monitors and other types of video devices. The LCD generates images according to video signals by controlling the transmission of light through the LCD. Thus, an LCD requires a light source, such as a backlight, because the LCD itself does not emit light.
The types of light sources used for a backlight include a halogen cathode fluorescent tube and a cold cathode fluorescent tube (CCFL). The CCFL is a light tube that emits light through a cold emission phenomenon such that electrons are emitted from the surface of a cathode when strong electric fields are applied across the light tube. The CCFL is used in applications that require low heat emission, high brightness, long lifetime and full color. A backlight that uses a CCFL can either be a light waveguide type backlight, a direct incident type backlight or a reflection type backlight.
The CCFL can either be an internal electrode fluorescent lamp in which electrodes are formed inside the light tube or an external electrode fluorescent lamp in which electrodes are formed outside the light tube. Such fluorescent lamps are driven by a high voltage AC waveform. The high voltage AC waveform for driving the fluorescent lamps is obtained by a DC voltage from a DC voltage source being converted to a low voltage AC waveform by an inverter and then the low voltage AC waveform is stepped up to a high voltage AC waveform by a transformer.
FIG. 1 illustrates a schematic block diagram of an apparatus for driving a backlight according to the related art. As shown in FIG. 1, the apparatus for driving a backlight of the related art includes a plurality of lamps 10, each of which has first and second electrodes. A transformer 20 provides a high voltage AC waveform to drive the plurality of lamps 10 in parallel. A shutdown circuit 30 generates a shutdown signal voltage Vsd based on the high voltage AC waveform output from the transformer 20. A controller 40 converts an input voltage Vin into an AC waveform that is provided to the transformer 20 and performs shutdown of the AC waveform being provided to the transformer 20 in response to the shutdown signal from the shutdown circuit. When the AC waveform being provided to the transformer 20 is shutdown, the high voltage AC waveform is shutdown to the plurality of lamps 10.
The plurality of lamps 10 emits light in response to the high voltage AC from the transformer 20. The first electrode of each of the plurality of lamps 10 is commonly connected to an output lead N1 of the transformer 20, and the second electrode of each of the plurality of lamps 10 is electrically grounded to the earth. A first capacitor Ca is electrically located between each first electrode of the plurality of lamps 10 and the output lead N1 of the transformer 20, and a second capacitor Cb is electrically connected between each second electrode of the plurality of lamps 10 and the ground source. The first and second capacitors Ca and Cb balance currents among the plurality of lamps 10 to prevent non-uniformity of brightness among the plurality of lamps 10.
The transformer 20 includes a primary coil L1 receiving an AC waveform from the controller 40 and a secondary coil L2 commonly connected to the plurality of lamps 10. The transformer 20 raises the AC waveform input via the primary coil L1 such that a high voltage AC waveform can be induced in the secondary coil L2 according to a winding ratio of the first coil L1 and secondary coil L2. Therefore, the high voltage AC waveform induced in the secondary coil L2 is output via the output lead N1 to the first electrode of each of the lamps 10.
The controller 40 converts an input voltage Vin provided from an external source into an AC waveform and then provides the AC waveform to the primary coil L1 of the transformer 20. The controller 40 includes at least one switching element (not shown) for converting the input voltage Vin to the AC waveform and a switching controlling unit (not shown) for controlling the switching element. The switching element is switched according to a switching control signal of the switching controlling unit to convert the input voltage Vin to the AC waveform and then to provide the AC waveform to the primary coil L1 of the transformer 20. The switching controlling unit generates such a switching control signal to control the switching element, and turns off the switching element according to the shutdown signal provided from the shutdown circuit 30, to cut off the high voltage AC waveform provided to the primary coil L1 of the transformer 20, thereby performing shutdown of the apparatus for driving a backlight.
The shutdown circuit 30, which is electrically connected to the output lead N1 of the transformer 20, generates a shutdown signal Vsd, in response to the high voltage AC waveform which is provided to the plurality of lamps 10, and then provides the shutdown signal to the controller 40. Namely, the shutdown circuit 30 generates the shutdown signal voltage Vsd to perform shutdown of the apparatus for driving a backlight in response to voltage variations caused by abnormal operation of the plurality of lamps 10.
FIG. 2 illustrates a detailed circuit diagram illustrating a shutdown circuit of FIG. 1. As shown in FIG. 2, the shutdown circuit 30 includes a voltage detection circuit 32 located between the output lead N1 of the transformer 20 and the ground source, and a comparator 34 located between the voltage detection circuit 32 and the controller 40. The voltage detection circuit 32 includes first and second capacitors C1 and C2, which are serially connected between the output lead N1 and the ground source, and first and second diodes D1 and D2, which are located between a voltage divider node N2 and the comparator 34, in which the voltage divider node N2 is between the first and second capacitors C1 and C2. The first and second capacitors C1 and C2 serve to smooth the voltage corresponding to the high voltage AC waveform output from the output lead N1 of the transformer 20, and to perform voltage division such that a divided voltage can be output to the voltage divider node N2.
The anode of the first diode D1 is electrically connected to the voltage divider node N2 and the cathode thereof is connected to the comparator 34. The cathode of the second diode D2 is connected to the voltage divider node N2 and the anode thereof is connected to the ground source. Such first and second diodes D1 and D2 rectify the voltage of the voltage divider node N2 and provide the rectified detection voltage to the comparator 34.
The comparator 34 includes first and second input leads, in which the first input lead is electrically connected to the cathode of the first diode D1 to input the detection voltage Vsen thereto, and the second input lead is electrically connected to the ground source to input voltage Vref thereto. Such a comparator 34 compares the detection voltage Vsen provided through the first input lead to the reference voltage Vref provided through the second input lead and then generates the shutdown signal voltage Vsd, which is provided to the controller 40. More specifically, the comparator 34 generates an OFF state shutdown signal if the detection voltage Vsen is greater than the reference voltage Vref or an ON state shutdown signal if the detection voltage Vsen is less than the reference voltage Vref. Such a shutdown circuit 30 detects voltage variation of the high voltage AC waveform, which is provided to the lamps 10, and then provides an OFF state shutdown signal to the controller, if each of the plurality of lamps 10 is normally driven. On the other hand, if each of the lamps 10 is abnormally driven, such as an open operation, the shutdown circuit 30 provides an ON state shutdown signal to the controller 40.
The apparatus for driving a backlight according to the related art converts the input voltage Vin to a high voltage AC waveform using the switching element of the controller 40 and the transformer 20, and then provides the AC high voltage to the plurality of lamps 10 in parallel such that each of the lamps 10 can be lit. While the plurality of lamps 10 is driven in parallel by the high voltage AC waveform, the detection voltage Vsen responds to the voltage variation of the high voltage AC waveform output from the transformer 20 and the shutdown circuit 30 provides the ON state shutdown signal to the controller 40 if the detection voltage Vsen is greater than the reference voltage Vref. Therefore, the controller turns off the switching element in response to the ON state shutdown signal such that the apparatus for driving a backlight can be shut down.
The detection voltage Vsen detected by the shutdown circuit 30 is affected by the electrical states of the plurality of lamps 10, such as open, short, and abnormal light. However, the detection voltage Vsen may be sensitive to peripheral temperature and/or changes of characteristics of circuit elements, which are included in the shutdown circuit 32. Therefore, although the plurality of lamps 10 are operating normally, the detection voltage Vsen can be affected by a peripheral temperature change and/or change of characteristics of the circuit elements in the shutdown circuit 32 such that the shutdown circuit 30 recognizes the plurality of lamps 10 to be in an abnormal state and then generates the ON state shutdown signal to shut down the backlight when the backlight should not be shut down. In another circumstance, when one of the plurality of lamps 10 is opened, the detection voltage Vsen can be affected by a peripheral temperature change and/or change of characteristics of the circuit elements in the shutdown circuit 32 such that the shutdown circuit 30 recognizes the plurality of lamps 10 to be operating normally and then generates the OFF state shutdown signal to maintain operation of the backlight when the backlight should be shut down. Consequently, the apparatus for driving a backlight of the related art has disadvantages in that it cannot correctly detect a state of the plurality of lamps 10 due to malfunction of the shutdown circuit 30, which is caused by change of peripheral temperature or change of characteristics of circuit elements in the shutdown circuit 32.